1. Field of the Invention
The present invention relates to a method of growing lutetium aluminum perovskite crystals and in particular to a method for growing such crystals for use as scintillators. The invention also provides improved scintillation detectors incorporating such crystals.
2. Description of the Prior Art
Cerium doped lutetium aluminum perovskite crystals (LuAlO.sub.3 :Ce, hereinafter "LuAP") are known to be useful as scintillators for .gamma. ray and x-ray detectors. Reports of the use of LuAP as a scintillator and its characteristics can be found in A. O. Ivanov et al, Opt. Spectrosc., Vol. 38, No. 2, February 1975 (230-232), G. V. Anan'eva et al, Sov. Phys. Crystallogr. 23(1), January-February 1978, I. A. Bondar et al, Dokl. Akad. Nauk. SSSR 246 (1979) 132, A. G. Petrosian, Journal of Crystal Growth 139 (1994) 372-392, W. W. Moses et al, IEEE Trans. Nucl. Sci. November 1994, A. Lempicki et al, IEEE Trans. Nucl. Sci. 42, August 1995, 280-284, M. Moszynski et al, (LuAP, A New Fast Scintillator) Preprint--Proceedings of SCINT 95 Conference, August 1995, Delft, The Netherlands, A. G. Petrosyan et al, (Preparation of Single Phase LuAlO.sub.3 --Ce Scintillator Crystals) Preprint--Proceedings of SCINT 95 Conference, August 1995, Delft, The Netherlands, and A. Lempicki et al (Lutetium Aluminate: Spectroscopic and Scintillation Properties) paper presented at IEEE Nucl. Sci. Symp., October 1995, San Francisco, USA
One particular use of crystal scintillators is in the field of borehole logging in which measurements of .gamma. rays (and x rays) are used to determine properties of underground formations surrounding a borehole. In such applications, the amount of space available for detectors is often limited and so it is not possible to use a large number of small crystal scintillators, each with its own photomultiplier as is done for example in medical imaging applications. Thus for borehole logging applications, it is desirable to use one or a few relatively large, single crystal scintillators which have good light output. While LuAP shows properties which would make it a likely candidate for use as a scintillator for borehole applications, it has heretofore been impossible to grow crystals large enough to provide the performance required for such applications. In all of the prior art referenced above, the largest samples reported are of the order of &lt;0.1 cm.sup.3 which is too small to be effective in borehole applications.
U.S. Pat. No. 4,958,080 and U.S. Pat. No. 5,025,151 (both incorporated herein by reference) describe a single crystal scintillator formed from a melt of formula Ce.sub.2x Lu.sub.2(1-x) SiO.sub.5, wherein 2.times.10.sup.-4 &lt;.times.&lt;3.times.10.sup.-2 (hereinafter the term LSO will be used as shorthand for this formula). The method proposed for growing LSO is the Czochralski crystal growth method which is generally described in the publication CRYSTAL GROWTH: A TUTORIAL APPROACH, W. Bardsley, D. T. J. Hurle, J. B. Mullins (eds) North-Holland Publishing Company, 1979, pp 189-215. This method has been used for growing scintillator crystals as is described in "Growth and applications of Gd2SiO5:Ce scintillators" T. Utsu and S. Akiyama, Journal of Crystal Growth 109 (1991) 385-391 for gadolinium orthosilicate scintillators and in "Czochralski growth of rare earth oxyorthosilicate single crystals" C. L. Melcher, R. A. Manente, C. A. Peterson and J. S. Schweitzer, Journal of Crystal Growth 128 (1993) 1001-1005, and "A Macintosh-based System For Czochralski Crystal Growth" C. L. Melcher, R. A.Manente, C. A. Peterson, J. S. Schweitzer, M. A. Singelenberg and F. J. Bruni, Scientific Computing and Automation, January 1994 39-45 for LSO scintillators. The Czochralski method can also be used to grow LuAP.
While this method can be used to make scintillator crystals which are useful in certain applications, there is variability between the scintillation behavior of different crystals cut from a single boule of LSO or LuAP. This variability has not been seen in any other property than the scintillation behavior, i.e. to all other intents and purposes, the boule appears to be homogenous, but the resulting crystals have .gamma.-ray spectra with broad or multiple peaks. However, for spectroscopic uses of LSO or LuAP crystal scintillators, it is desirable that the scintillation behavior be as uniform as possible.
It is an object of the present invention to provide a method for producing LuAP crystals for use as scintillators which display substantially uniform scintillation behavior throughout the crystal.
It is another object of the present invention to provide an improved Czochralski crystal growth method for producing LuAP crystals.